Biochar, more commonly known simply as charcoal, is the solid carbon residue which results from the pyrolysis of various biomass feedstocks such as carbonaceous materials having a cellulosic fibrous structure. Such materials include various types of wood, vegetable biomass, tree bark, paper, paperboard, cardboard, certain waste products, and the like. Pyrolysis is the chemical transformation or decomposition of chemical compounds caused by heat. It occurs spontaneously at sufficiently high temperatures, typically in the range of 300° to 800° C., and produces certain by-products such as carbonized biomass, combustible gases and volatile organic compounds (VOC's) known as tars.
Pyrolysis has been used for centuries as a method for producing carbonized materials. The process is typically carried out in a kiln or in a retort. A kiln is a thermally insulated chamber in which the oxygen available for combustion is restricted. A retort is almost as simple as the kiln, involving an outer space where pyrolysis products are burned providing energy to a central space where biomass is pyrolysed without oxygen. Both the kiln and retort are simple to construct but time intensive to produce char, and both require loading and unloading between production cycles. Moreover, without adequate flaring or burn-off of gaseous by-products, these methods can produce a large amount of air pollutants.
Two primary methods for pyrolysis exist that pertain to producing biochar-fast pyrolysis and slow pyrolysis. A number of other methods, such as microwave pyrolysis and vacuum pyrolysis, exist but are not directly relevant to this application. Although pyrolysis has been the leading producer of biochar, the focus of most pyrolysis operations has been to produce pyrolysis gases and/or oils with charcoal as a byproduct for use as a fuel or agricultural amendment.
Because fast and slow pyrolysis requires an oxygen free environment, the process must be carried out in a closed vessel. When the vessel fills with pyrolysis gas (primarily H2, CO2 and CH4) a significant explosion hazard is created which demands a higher engineering and production cost to ensure the safety of personnel working with the equipment. In addition, one of the primary by-products of pyrolysis is an oil composed of various compounds including high percentages of oxygenated hydrocarbons and poly-nuclear aromatics. These oils can be hazardous to tissues and represent a substantial environmental hazard.
The art is replete with examples of apparatus for producing charcoal. An early example is U.S. Pat. No. 757,939 issued Apr. 19, 1904 to Mackie for “Apparatus for the distillation of Wood.” Mackie discloses an inclined retort contained within a kiln and adapted for pyrolysis of wood. The retort includes a drain pipe at one end thereof for collecting the pyrolysis by-products and condensing them in a suitable collector. However, as can be seen from the configuration of the system, it is a batch-type operation which requires loading, sealing, heating, unsealing and unloading the retort-a very time consuming process.
Another example is found in U.S. Pat. No. 6,790,317 issued Sep. 14, 2004 to Anatal for “Process for Flash Carbonization of Biomass.” Anatal discloses a process for the low energy input pyrolytic conversion of biomass in an atmosphere of pressurized air. Again, the process is not continuous, and requires of loading and unloading of material between cycles. Moreover, the process disclosed by Anatal produces a product charcoal that has a higher content of volatiles than a biochar produced at atmospheric pressure.
Considerable work has gone into the improvement of production systems and techniques for manufacturing a fuel gas for use in the generation of heat or electricity. Biomass gasification is the process of thermally decomposing organic material into a combustible gas composed primarily of H2 and CO2 with a mixture of CH4, higher hydrocarbons and nitrogen. For example, U.S. Pat. No. 4,583,992 issued Apr. 22, 1986 to Rogers discloses a system for the co production of gas and charcoal for heating. Co-production relies on a rotating grate at the bottom of the reactor to pulverize char and support the reaction, the inclusion of ceramic balls at the grate, and the introduction of air through rotating tuyeres. The focus of Rogers' system is on increasing the heating value of the input feedstock by decomposition into gas and charcoal.
U.S. Pat. No. 6,647,903 issued Nov. 18, 2003 to Ellis discloses a co-current downdraft gasifier system for the gasification of biomass and charcoal. Nonetheless, the apparatus of Ellis' invention is not focused on the production of charcoal, but rather on the introduction of oxygen into the region of hot charcoal to increase temperatures above 1000° C., thereby destroying volatile hydrocarbons in the produced gas. This system consumes the majority of char produced in order to produce a low tar gas. At the same time, input material must be of a well controlled size and moisture, along with being free of rocks or sand. Because of high reactor temperatures, the material of construction must be specified to withstand high heat.
The primary goal of gasification has been to produce a fuel gas for use in the generation of heat or electricity. While chars have been produced by gasification systems in the past, it has been a considered waste product, and substantial work has been done to reduce the percentage of char produced by a gasification unit. Further, the char produced by gasification units is generally intended to be used as a fuel and is made at a higher temperature than the temperature required to manufacture char for use as an agricultural amendment, thereby wasting fuel resources. Accordingly, it would be advantageous to develop a gasification system and methodology which would result in a char that can be used not only as an agricultural amendment, but which also possess physical properties such as energy content, ash content and friability that would allow it to serve as a coal substitute.
While gasification systems are able to accept a wide variety of materials, there are few operational systems that can run well for continued periods of time on a broad range of input material. Frequently feedstocks for gasification require substantial cleaning and screening before they can be used in the system. Feedstock materials must also be dried thoroughly due to gasifiers' limited capabilities to deal with moisture above 7-10%. Because gasification occurs at temperatures above 800° C. and frequently above 1100° C., gasification equipment construction materials must withstand elevated temperatures, substantially increasing gasification system engineering and construction costs.
Air quality is also a primary concern. Frequently, especially in small scale operations or in retorts, the pyrolysis process produces a large mass of gases that can be considered air pollutants, specifically CH4 and higher hydrocarbons. If these gases are not properly combusted, they can present a significant air quality and greenhouse gas risk. Many gasification and pyrolysis units are operated either at such a small scale (camp stoves, two barrel retort) that production of a substantial amount of char for a commercial user is impractical or at such a large scale that feedstocks must be transported over substantial distances to the processing facility. At the end of the production cycle, the carbonized material must also be transported to the end use site. In such cases, the materials transportation costs quickly become prohibitively expensive, or the CO2 emissions of transportation become larger than the carbon sequestered by the end product.
In view of the foregoing, it is apparent that a need exists for a process and apparatus which is capable of safely, economically and continuously producing carbonaceous and gaseous pyrolysis by-products for soil amendment, fuel and other applications from a wide variety of feedstocks and which can be transported to sites where the feedstock can be collected or to sites where the biochar may be needed, or both.